I've got a 64S, and my buddy has a 64ST.
Don't waste your money on the 64ST. The included topo maps are useless.
Don't waste your money on the 64S. S theoretically stands for "sensor", which means barometer and compass. Neither of which work AT ALL in my unit, or my buddies. The unit has no idea how high I am, and has no idea which direction I'm facing while I stand still. Worthless.
The 64 base model may be OK, but remember Garmin deleted weatherproof power cord inputs a few years ago, so when mine was plugged in on the quad, I had to put a bread bag over it. So lame.
In both cases, the UI is similar to a quality, polished smartphone app in the same way that the aerodynamic qualities of a hummingbird and a cinder block are similar. 1980s clunky.
The 64 comes up for sale on Amazon periodically.
I was getting (pre power cord) 4-5 days of 6-8 hours per day use out of the 64S with those fancy non-rechargeable lithium batteries.